The best tip to give in this game is this: DO NOT HAGGLE! Money isn't the only consideration here: you need to increase your merchant levels to get access to better stuff, and increase customer reputation (indicated by the little hearts you may see during transactions) so that customers have bigger budgets. Both are related to good client relations, which effectively translates to no haggling, ever. More specifically, By selling near 105-110%, you'll get pin and conbo bonusses by the truckload. If you do this, you should easily make 100k in a day in the last week, perhaps even in a single timeslot.

Another thing to be aware of is Customer stats to get a feel of the customers budgets. Also keep track of the hearts. The first heart for a customer increases the customers budget by 10x, so that's kind of a big deal. Later hearts increase it by 10%-ish. Doing this let's you gauge what you can sell things for.

Know the item categories[recettear.wikia.com] so you know what to seel when they ask for a category, and the announcements[recettear.wikia.com] to stack up on XP during a boom.